This invention relates to a plastic foam material. More specifically, this invention is directed to a plastic foam material composed of a polyolefin based resin composition (hereinafter referred to as "plastic foam material") and products derived therefrom. The present invention is also directed to a method of making the plastic foam material.
Due to their superior heat insulating properties, prior art plastic foam material has found widespread use as aircraft, train and automobile components such as panels and seating.
The aforementioned products and interior components can be formed by various means, including vacuum molding, press molding, and similar processes at high temperature. Accordingly, most plastic foam materials composed of a polyolefin based resin contain a polypropylene based resin, which imparts superior heat resistance properties to products derived from the plastic foam material.
Polypropylene based resin and products derived therefrom, however, are plagued by numerous drawbacks. Chief among them is a tendency to deteriorate. This often happens because with polypropylene based resin and derived products, the involved molecular chains are subject to breakage during the foaming process. Such deterioration results from the stresses produced by ionizing radiation, peroxides and similar means.
These stresses on the molecular chains occur during cross-linking and during the foaming process. Such breakage encountered by polypropylene based resins during the foaming process is a problem which has not been adequately addressed in the prior art.
In an attempt to overcome the aforementioned deficiency related to prior art polypropylene based resins and derived products, the use of a polyethylene based resin has been proposed. Employing such a polyethylene based resin was attempted in order to strengthen the resulting products. The goal was to eliminate breakage encountered by polypropylene based resins in making plastic foam materials composed of polyolefin based resins and derived products.
One such method, disclosed in Japanese Laid Open Patent Publication S46-39716, attempts to overcome the numerous disadvantages of prior art plastic foam materials using a method for making plastic foam material composed of polyolefin based resin, which includes a foaming agent. The foaming agent is added after cross-linking the polyolefin based resin sheet with ionizing radiation. However, the moldability of the plastic foam material derived from this process is inadequate.
Similarly, Japanese Laid Open Patent Publication H2-102234 discloses a plastic foam material composed of a polyolefin based resin wherein the resin composition used to form the plastic foam material includes a combination of a polyethylene based resin and a polypropylene based resin. The degree of cross-linkage of the polyethylene based resin is essentially the same as that of the polypropylene based resin.
However, the moldability of the resulting plastic foam material composed of the aforementioned mixed resin composition is severely compromised.
The prior art also shows attempts to improve the moldability of plastic foam materials composed of polyolefin based resins. Japanese Laid Open Patent Publication No. S63-1977 discloses a process wherein the plastic foam material, composed of a polyolefin based resin, is formed from a polyolefin resin in which the degree of cross-linkage in the inner layers is higher than that in the surface layers.
However, the anticipated improvement disclosed in the process of Japanese Laid Open Patent Publication No. S63-1977 does not achieve adequate cross-linkage in the inner layers. Therefore, the improvement in the moldability of the resulting plastic foam material is insufficient.
Generally, polypropylene based cross-linked resin is used in similar ways according to the prior art processes. This includes lining the inside surface of a product with a polypropylene based cross-linked resin by vacuum molding or press forming.
Such surface materials generally include polyvinyl chloride sheets, thermoplastic elastomer sheets, natural or artificial fabrics, leather and similar materials, according to the prior art processes. The temperature for vacuum molding and press forming is generally higher than 160.degree. C.
However, breakage readily occurs due to the mismatch of the elongation rate of the plastic foam material with the elongation rate of the surface material at high temperature. This is because the elongation rate of the plastic foam material at high temperature is considerably slower than that of the surface materials.
Additionally, the resulting plastic foam material is liable to break and disintegrate if its heat-resistance is inadequate. As a result, the top layer of the plastic foam material is liable to peel off, causing the surface layer to swell, which in turn, may allow creases to appear on the surface of the layered body.
Japanese Laid Open Patent Publication No. H2-67129 proposes joining a plastic foam material composed of a polyolefin based resin with its surface material by adjusting the fluidity of the resin. This is done with a view towards solving the above enumerated problems associated with the prior art processes. This fluidity adjustment is done using a polyolefin based resin with a melt index (MI) which falls within a specified range.
The desired MI requires a specific temperature such as 230.degree. C. Japanese Laid Open Patent Publication No. H2-67129 does not disclose good moldability and adherence at other operation temperatures.
The use of a mixed resin composition containing polypropylene based resin and polyethylene based resin has been studied in order to substantially improve heat-resistance of plastic foam :materials composed of a polyolefin based resin.
Japanese Laid Open Patent Publications Nos. S46-38716 and No. S58-57542 each disclose the use of specific cross-linking agents to achieve better cross-linkage. Similarly, Japanese Laid Open Patent Publication No. S63-1977 discloses use of a specified amount of cross-linkage in an attempt to improve cross-linking in prior art plastic foam materials.
However, none of the prior art references manages to overcome the problem of the compatibility between cross-linking agents and involving specific resin components. Additionally, substantial differences exist between the dispersion of cross-linking agents in each of the various resin components. The difficulty in achieving desired dispersion increases as the degree of cross-linkage between various cross-linking agents and resin components increases.